Patent GB-864977-A (GB)
Title: Improved radio receiver apparatus
IPC: H03D1/18
Filed: 1957-11-07
Abstract: 864,977. Radio receiving circuits employing transistors. GENERAL MOTORS CORPORATION. Nov. 7, 1957 [Nov. 13, 1956], No. 34705/57. Classes 40(5) and 40(6) The detector of a "stop on signal" receiver has a diode in series with its input circuit to minimise the effects of changes of capacitance with changes in signal level. The invention is described in relation to a transistor superheterodyne car radio receiver in which facilities are provided for direct mechanical control of the tuning by push buttons as well as automatic control of the so called "stop on signall" type in which, upon the operation of a switch, the receiver tuning is driven to the next station. The H.F. amplifier (Fig. 1) comprises a transistor amplifier 2 having the aerial input winding 36 arranged at one end of the winding 42 so as to maintain constant sensitivity over the wave-range. The output of the transistor is connected to a signal selecting and image suppressor circuit 56, 58, 60, feeding the frequency changer transistor 4. The frequency changer (Fig. 1) comprises a transistor 4 having the signal applied to its base and the local oscillation applied to its emitter. The output circuit comprises a tuned winding 134 coupled by a capacitor 154 to tuned winding 148 there being no inductive coupling between the windings. The local oscillator comprises a transistor 6 (Fig. 1) having a tuned collector circuit 106 inductively coupled to a coil 98 in the base circuit. Stabilisation of the operating frequency is effected by operating the transistor at a high current level and by connecting a capacitor 120 between collector and emitter to swamp changes in the inter-electrode capacitance. In addition, the effect of changes in the collector supply voltage is compensated by changes in the emitter base bias. The oscillator is coupled to the frequency changer by a lead 88 connected to a capacitive potential divider 110, 112. The I.F. amplifier comprises transistors 8, 10 and 12 (Fig. 1a) coupled by single tuned circuits, the output of transistor 12 being coupled to the detector by a double tuned circuit 222, 244 (Fig. 1b) coupled through a capacitor 254, there being no inductive coupling between the coils. Transistor 216 is operated at a higher current than the earlier I.F. transistors. The detector (Fig. 1b) comprises a grounded collector circuit coupled to the winding 244 through a diode 248. The output is taken from the slider of a volume control potentiometer 280 the slider being connected by a capacitor 294 tothe primary winding of the audio amplifier input transformer 288. During tuning the transistor operates with the collector supply disconnected and the diode is provided to maintain the input capacity of the transistor more nearly constant with changes in signal level. The audio amplifier comprises a transistor 16 (Fig. 1b) having its collector connected to the primary winding of a transformer 312 (Fig. 1) feeding a push-pull amplifier 18, 20, a feedback winding 310 being connected back to the emitter circuit of the previous transistor 16. The pushpull amplifier is stabilised by means of a temperature variable resistor 378 and tone control is effected by a switch 326 which enables different circuits to be connected in shunt with a capacitor 320 connected across the input transformer of the push-pull stage. The output of this stage is fed through a transformer to a pair of volume controls 398 (Fig. 1b) feeding front and rear speakers. Automatic gain control is applied to the R.F. and first I.F. stages, the control voltage being derived from a rectifier 550 (Fig. 1b) connected to a coil 242 (Fig. 1a) coupled to the last I.F. amplifier output circuit. The control voltage is applied to the base of a common emitter agc amplifier transistor 472 (Fig. 1a) having the collector load connected between emitter and earth. The amplified agc voltage is fed through a resistor 478 and a line 78 directly to the I.F. amplifier and through a diode 80 (Fig. 1) to the R.F. amplifier, the diode serving to delay the application of the voltage to the latter amplifier. Automatic tuning. On momentary operation of a switch 406 (Fig. 1b) a relay 26 is operated and this causes a relay 27 to operate. Contacts of relay 27 apply a bias to a cross coupled npn-pnp trigger circuit 22, 24 making both transistors conductive so as to hold relay 26 operated. A further contact of relay 27 disconnects the agc circuit and connects the output of the agc detector to the base of the trigger circuit transistor 22. A further contact disconnects the collector supply of the second detector transistor 14 so that the transistor operates as a diode, and the output of this is replied in the opposite polarity to the agc voltage to the base of trigger transistor 22. The voltage for the agc detector is derived from a winding coupled to the input winding of a double tuned circuit exhibiting a double humped input current characteristic so that as the tuning approaches a station the resulting control voltage rises and then falls. When the correct tuning position is reached the output from the detector exceeds the combined bias and control voltage applied to transistor 22, the transistors of the trigger circuit accordingly cut off. This releases relay 26 thus releasing relay 27, so that a detent operated by relay 27 engages the tuner drive and stops the tuning. The tuning cores are mounted on a tuning carriage and tuning is effected under the action of a spring. At the limit of movement of the carriage, limit switches operate an electro-magnet which returns the carriage to the beginning of its movement, a further electro-magnet being provided to recharge the spring when necessary. A variable resistor 142 (Fig. 1) permits the sensitivity of the receiver to be adjusted during automatic tuning so as to determine the level of signal that will cause tuning to be arrested. Specification 540,542 is referred to.